How are the high temperatures for this type of battery maintained? Do they have to have heaters to keep the sodium molten? And if so, does that not make it very inefficient? If the cell is allowed to cool so that the sodium and/or sulfur solidify, is that the end of it or can it be restarted?

Some applications that aren't naturally hot have a heater to get it started, and then are such high drain that the losses keep it hot enough (with thermal insulation). Other applications are already in a hot enough environment, such as in a spaceborne deployment for example, where the sun keeps it going.

Yes, heaters makes it inefficient -- but you can amortize the loss by not thermal cycling (re-starting) as much as possible, and by making the thermal insulation very effective for cases where self-heating doesn't work by itself.

In applications where the battery goes cold, the design is such that the restart to operating temperatures works, with some lifecycle/servicelife penalty.

Thanks for this series of article! I did not know about so many chemistries!

I heard about sodium batteries which were installed in buses for the city of Quebec, Canada. A few years ago, one caught on fire and fortunately, no one got hurt. The manufacturer told city maintenance the battery was too old and needed replacement. Is this a "typical behavior" for such a battery?

No, it is rare. Whenever there are alot of anything in use, the probability of the rarities happening once, increase. Also, there probably was no mention of the 'abuse' history of the particular battery that failed.

Since the sodium must be kept molten for the battery to operate, it would seem that if the battery system is ever going to be shutdown it would benefit from a "bootstrapping" system. A small battery could be brought up first up with help from the charger system and then used to help bring up the first full size battery which would then help jump start the next battery until all the batteries were running again.

I have heard of this being done with other technologies. Another possibility, is to keep capacitors trickle charged, to quick-heat/dump, so that the bootstrapper doesn't have wear-out and chemistries issues.